Use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed the maximum file size (1 MB).

I'm not seeing this.
Reset the page zoom on pages that cause problems: <b>View > Zoom > Reset</b> (Ctrl+0 (zero); Cmd+0 on Mac)
* http://kb.mozillazine.org/Zoom_text_of_web_pages
Can you attach a screenshot?
* http://en.wikipedia.org/wiki/Screenshot
Use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed the maximum file size (1 MB).

Are you zooming the page?
I see those extra px appearing like in your screenshot with some specific zoom factors, but not if I reset the page zoom.
See also:
* https://developer.mozilla.org/en/Images%2C_Tables%2C_and_Mysterious_Gaps